The present invention relates to electrophotographic toners. More particularly, the present invention relates to electrophotographic toners having stable triboelectric properties and methods of printing images using these toners.
The image quality currently available is generally good in that prints have high solid area reflection density, low background in non-image areas, and consistent print quality from toner lot to toner lot and from the start of a new developer until it is replaced. The present toners, however, are not as good with respect to fusing quality and toner ruboff (e.g., the abrasion resistance of the fused image).
In attempting to improve toner ruboff, a wax, for instance, can be included in the toner. However, waxes can effect triboelectric properties of a toner. If the triboelectric properties are increased, the resulting prints may look gray because less toner is being transferred onto the paper. In addition, the toners may not be as free flowing as desired.
Accordingly, new toner formulations which provide an improved or reduced ruboff without affecting the charge and/or flow properties would be beneficial to those in the industry.
A feature of the present invention is to provide an electrophotographic toner having stable triboelectric properties.
Another feature of the present invention is to provide a toner formulation that has improved ruboff properties.
A further feature of the present invention is to provide an electrophotographic toner formulation that reduces ruboff and yet provides satisfactory charge and/or flow properties.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the written description and appended claims.
To achieve these and other advantages and in accordance with the purposes of the present invention as embodied and broadly described herein, the present invention relates to a toner formulation containing at least one toner resin, at least one first charge control agent capable of providing a consistent or controllable level of charge, at least one second control agent capable of providing a sustained level of charge, and optionally at least one surface treatment agent, and optionally at least one release agent, and optionally at least one colorant.
The present invention further relates to a method of decreasing toner ruboff on an image and involves printing an image on a substrate using the above-identified toner formulation of the present invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide a further explanation of the present invention, as claimed.
The present invention relates to toner formulations preferably having stable triboelectric properties and also preferably having acceptable toner ruboff properties. Further, preferably the toner formulations of the present invention are free flowing. In more detail, the toner formulations of the present invention contain at least one toner resin, at least one first charge control agent capable of providing a consistent level of charge and at least one second charge control agent capable of providing a sustained level of charge. The toner formulation preferably further contains at least one surface treatment agent which is optional and optionally at least one release agent. Optionally, the toner formulation can contain at least one colorant and other conventional components typically found in toner formulations.
The toner formulations of the present invention can be used in single component toners or two component toners. Preferably, the toner formulations of the present invention are used in two component toner/developer systems.
In the present invention, the toner resin can be any conventional polymeric resin or combination of resins typically used in toner formulations using conventional amounts.
The toner particles can include one or more toner resins which can be optionally colored by one or more colorants by compounding the resin(s) with at least one colorant and any other ingredients. Although coloring is optional, normally a colorant is included and can be any of the materials mentioned in Colour Index, Volumes I and II, Second Edition, incorporated herein by reference. The toner resin can be selected from a wide variety of materials including both natural and synthetic resins and modified natural resins as disclosed, for example, in U.S. Pat. Nos. 4,076,857; 3,938,992; 3,941,898; 5,057,392; 5,089,547; 5,102,765; 5,112,715; 5,147,747; 5,780,195 and the like, all incorporated herein by reference. Preferred resin or binder materials include polyesters and styrene-acrylic copolymers. The shape of the toner particles can be any shape, regular or irregular, such as spherical particles, which can be obtained by spray-drying a solution of the toner resin in a solvent. Alternatively, spherical particles can be prepared by the polymer bead swelling techniques, such as those described in European Patent No. 3905 published Sep. 5, 1979, which is incorporated in its entirety by reference herein.
Typically, the amount of toner resin present in the toner formulation is from about 85 to about 95% by weight of the toner formulation.
The term xe2x80x9ccharge-controlxe2x80x9d refers to a propensity of a toner addendum to modify the triboelectric charging properties of the resulting toner. A very wide variety of charge control agents for positive and negative charging toners are available. Suitable charge control agents are disclosed, for example, in U.S. Pat. Nos. 3,893,935; 4,079,014; 4,323,634; 4,394,430; and British Patent Nos. 1,501,065 and 1,420,839, all of which are incorporated in their entireties by reference herein. Additional charge control agents which are useful are described in U.S. Pat. Nos. 4,624,907; 4,814,250; 4,840,864; 4,834,920; 4,683,188; and 4,780,553, all of which are incorporated in their entireties by reference herein. Mixtures of charge control agents can also be used. Particular examples of charge control agents include chromium salicylate organo-complex salts, and azo-iron complex-salts, an azo-iron complex-salt, particularly ferrate (1-), bis[4-[(5-chloro-2-hydroxyphenyl)azo]-3-hydroxy-N-phenyl-2-naphthalenecarboxamidato(2-)], ammonium, sodium, and hydrogen (Organoiron available from Hodogaya Chemical Company Ltd.).
With respect to the first charge control agent, as indicated above, one or more different types of first charge control agents can be used. The first charge control agent is capable of providing a consistent level of charge (e.g., controllable level of charge). The first charge control agent has the ability to xe2x80x9cdial inxe2x80x9d the desired charge level. For purposes of the present invention, a preferred consistent level of charge is from about xe2x88x9210 to about xe2x88x9230 micro C/gm. The toner Q/m ratio can be measured in a MECCA device comprised of two spaced-apart, parallel, electrode plates which can apply both an electrical and magnetic field to the developer samples, thereby causing a separation of the two components of the mixture, i.e., carrier and toner particles, under the combined influence of a magnetic and electric field. A 0.100 g sample of a developer mixture is placed on the bottom metal plate. The sample is then subjected for thirty (30) seconds to a 60 Hz magnetic field and potential of 2000 V across the plates, which causes developer agitation. The toner particles are released from the carrier particles under the combined influence of the magnetic and electric fields and are attracted to and thereby deposit on the upper electrode plate, while the magnetic carrier particles are held on the lower plate. An electrometer measures the accumulated charge of the toner on the upper plate. The toner Q/m ratio in terms of microcoulombs per gram (xcexcC/g) is calculated by dividing the accumulated charge by the mass of the deposited toner taken from the upper plate.
Examples of suitable first charge control agents include, but are not limited to, acidic organic charge control agents. Particular examples include, but are not limited to, 2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-one (MPP) and derivatives of MPP such as 2,4-dihydro-5-methyl-2-(2,4,6-trichlorophenyl)-3H-pyrazol-3-one, 2,4-dihydro-5-methyl-2-(2,3,4,5,6-pentafluorophenyl)-3H-pyrazol-3-one, 2,4-dihydro-5-methyl-2-(2-trifluoromethylphenyl)-3H-pyrazol-3-one and the corresponding zinc salts derived therefrom. Other examples include charge control agents with one or more acidic functional groups, such as fumaric acid, malic acid, adipic acid, terephathalic acid, salicylic acid, fumaric acid monoethyl ester, copolymers of styrene/methacrylic acid, copolymers of styrene and lithium salt of methacrylic acid, 5,5xe2x80x2-methylenedisalicylic acid, 3,5-di-t-butylbenzoic acid, 3,5-di-t-butyl-4-hydroxybenzoic acid, 5-t-octylsalicylic acid, 7-t-butyl-3-hydroxy-2-napthoic acid, and combinations thereof. Still other acidic charge control agents which are considered to fall within the scope of the invention include N-acylsulfonamides, such as, Nxe2x80x94(3,5-di-t-butyl-4-hydroxybenzoyl)-4-chlorobenzenesulfonamide and 1,2-benzisothiazol-3(2H)-one 1,1-dioxide.
The first charge control agent is generally present in the toner formulation in an amount to provide a consistent or controllable level of charge and preferably provide a consistent level of charge of from about xe2x88x9210 to about xe2x88x9230 micro C/gm in the toner formulation upon being charged. This level is especially preferred when the toner particle size is large, such as from about 10 microns to about 12 microns. Examples of suitable amounts include from about xc2xd part to about 3 parts per 100 parts of resin present in the toner formulation. Other preferred ranges for the consistent level of charge include from about xe2x88x9230 to about xe2x88x92120 micro C/gm in the toner formulation upon being charged. This is especially preferred when the toner particle size is small, such as from about 4 microns to about 8 microns.
Any acidic material having a pKa in the range of from about 2 to about 8 can be used as the first charge control agent. Acidic materials with pKa values of from 3 to 7 are particularly preferred. Acidic materials preferably contain a labile proton, such as a carboxylic, phenolic or sulfonic acid. More broadly an acid also includes compounds within the definition as proposed by G. N. Lewis, that is an acid is any material capable of accepting an electron pair from a donor molecule. The term pKa is defined as
pKa=xe2x88x92log(Ka) 
where Ka is an inherent molecular property of an acidic compound and is called the acid dissociation constant. Ka is defined by the following equation;   Ka  =                    [                  H          +                ]            ⁡              [                  B          -                ]                    [      HB      ]      
In a practical sense the lower the pKa value the stronger the acid. Acids useful in this invention may have more than one acid function per molecule such as di- and tri-carboxylic or di- and tri-sulfonic acids. Further, multiple types of acid functional groups may be present in the useful molecule for example as in p-carboxybenzenesulfonic acid. One skilled in the art can readily propose other such acids based on the foregoing definitions. Some monomeric acids useful in this invention along with their pKa values are given in Table 1.
With respect to the second charge control agent, one or more different types of second charge control agents can be used. The second charge control agent is capable of providing a sustained level of charge (e.g., stable level of charge) in the toner formulation. Preferably, a sustained level of charge is a triboelectric charge in the toner formulation that is sustained over time. For example and as shown in the Examples, for purposes of the present invention, preferably the second charge control agent(s) maintains a toner triboelectric charge of from about xe2x88x9210 to about xe2x88x9230 micro C/gm for a time period of from 2 minutes to 10 minutes. This sustained level of charge is especially preferred when the toner particle size is large, such as from about 10 microns to about 12 microns. Other preferred sustained levels of charge include from about xe2x88x9230 to about xe2x88x92120 micro C/gm in the toner formulation. This is especially preferred when the toner particle is small, such as from about 4 microns to about 8 microns. The sustained level of charge is measured by measuring the toner triboelectric charge in a developer mixture that has been agitated for 2 minutes and for 10 minutes.
A preferred class of second charge control agents include, but are not limited to, iron organo metal complexes such as organo iron complexes. A particular example is T77 from Hodogaya. The second charge control agent is generally present in the toner formulation in an amount such that the triboelectric charges in the toner are sustained for a period of from 2 minutes to 10 minutes within the range of xe2x88x9210 to xe2x88x9230 micro C/gm or from about xe2x88x9230 to about xe2x88x92120 micro C/gm. Other examples include metal salts of salicylic acid and metal salts of salicylic acid derivatives. Specific examples are set forth in U.S. Pat. No. 4,762,763, incorporated herein by reference. Examples of suitable amounts of the second charge control agent include, but are not limited to, from about xc2xd part to about 3 parts per 100 parts of toner resin in the toner formulation.
In the present invention, in some embodiments, at least one release agent is preferably present in the toner formulation. An example of a suitable release agent is one or more waxes. Useful release agents are well known in this art. Useful release agents include low molecular weight polypropylene, natural waxes, low molecular weight synthetic polymer waxes, commonly accepted release agents, such as stearic acid and salts thereof, and others.
The wax is preferably present in an amount of from about 0.1 to about 10 wt % and more preferably in an amount of from about 1 to about 6 wt % based on the toner weight. Examples of suitable waxes include, but are not limited to, polyolefin waxes, such as low molecular weight polyethylene, polypropylene, copolymers thereof and mixtures thereof. In more detail, more specific examples are copolymers of ethylene and propylene preferably having a molecular weight of from about 1000 to about 5000 g/mole, particularly a copolymer of ethylene and propylene having a molecular weight of about 1200 g/mole. Additional examples include synthetic low molecular weight polypropylene waxes preferably having a molecular weight from about 3,000 to about 15,000 g/mole, such as a polypropylene wax having a molecular weight of about 4000 g/mole. Other suitable waxes are synthetic polyethylene waxes. Suitable waxes are waxes available from Mitsui Petrochemical, Baker Petrolite, such as Polywax 2000, Polywax 3000, and/or Unicid 700; and waxes from Sanyo Chemical Industries such as Viscol 550P and/or Viscol 660P. Other examples of suitable waxes include waxes such as Licowax PE130 from Clarient Corporation.
With respect to the surface treatment agent also known as a spacing agent, the amount of the agent on the toner particles is an amount sufficient to permit the toner particles to be stripped from the carrier particles in a two component system by the electrostatic forces associated with the charged image or by mechanical forces. Preferred amounts of the spacing agent are from about 0.05 to about 5 weight percent, and more preferably from about 0.1 to about 3 weight percent, and most preferably from about 0.2 to 0.6 weight percent, based on the weight of the toner.
The spacing agent can be applied onto the surfaces of the toner particles by conventional surface treatment techniques such as, but not limited to, conventional powder mixing techniques, such as tumbling the toner particles in the presence of the spacing agent. Preferably, the spacing agent is distributed on the surface of the toner particles. The spacing agent is attached onto the surface of the toner particles and can be attached by electrostatic forces or physical means or both. With mixing, preferably uniform mixing is preferred and achieved by such mixers as a high energy Henschel-type mixer which is sufficient to keep the spacing agent from agglomerating or at least minimizes agglomeration. Furthermore, when the spacing agent is mixed with the toner particles in order to achieve distribution on the surface of the toner particles, the mixture can be sieved to remove any agglomerated spacing agent or agglomerated toner particles. Other means to separate agglomerated particles can also be used for purposes of the present invention.
The preferred spacing agent is silica, such as those commercially available from Degussa, like R-972, or from Wacker, like H2000. Other suitable spacing agents include, but are not limited to, other inorganic oxide particles and the like. Specific examples include, but are not limited to, titania, alumina, zirconia, and other metal oxides; and also polymer beads preferably less than 1 xcexcm in diameter (more preferably about 0.1 xcexcm), such as acrylic polymers, silicone-based polymers, styrenic polymers, fluoropolymers, copolymers thereof, and mixtures thereof.
When the toner formulation of the present invention is used in a two-component toner, the carrier particles used in association with the toner formulation can be conventional carrier particles. Thus, the carrier particles can be hard or soft magnetic carrier particles. With a two component developer, the toner concentration of the present invention is preferably present in an amount of from about 1 wt % to about 25 wt %, and more preferably from about 3 wt % to about 12 wt % based on the weight of the developer.
In more detail, the set up of the development system is preferably a digital printer, such as a Heidelberg Digimaster 9110 printer using a development station comprising a non-magnetic, cylindrical shell, a magnetic core, and means for rotating the core and optionally the shell as described, for instance, in detail in U.S. Pat. Nos. 4,473,029 and 4,546,060, both incorporated in their entirety herein by reference. The development systems described in these patents can be adapted for use in the present invention. In more detail, the development systems described in these patents preferably use hard magnetic carrier particles. For instance, the hard magnetic carrier particles can exhibit a coercivity of at least about 300 gauss when magnetically saturated and also exhibit an induced magnetic moment of at least about 20 EMU/gm when in an externally applied field of 1,000 gauss. The magnetic carrier particles can be binder-less carriers or composite carriers. Useful hard magnetic materials include ferrites and gamma ferric oxide. Preferably, the carrier particles are composed of ferrites, which are compounds of magnetic oxides containing iron as a major metallic component. For example, compounds of ferric oxide, Fe2O3, formed with basic metallic oxides such as those having the general formula MFeO2 or MFe2O4 wherein M represents a mono- or di-valent metal and the iron is in the oxidation state of +3. Preferred ferrites are those containing barium and/or strontium, such as BaFe12O19, SrFe12O19, and the magnetic ferrites having the formula MO.6 Fe2O3, wherein M is barium, strontium, or lead as disclosed in U.S. Pat. No. 3,716,630 which is incorporated in its entirety by reference herein. The size of the magnetic carrier particles useful in the present invention can vary widely, and preferably have an average particle size of less than 100 microns, and more preferably have an average carrier particle size of from about 5 to about 45 microns.
In a typical manufacturing process, the desired polymeric binder for toner application is produced. Polymeric binders for electrostatographic toners are commonly made by polymerization of selected monomers followed by mixing with various additives and then grinding to a desired size range. During toner manufacturing, the polymeric binder is subjected to melt processing in which the polymer is exposed to moderate to high shearing forces and temperatures in excess of the glass transition temperature of the polymer. The temperature of the polymer melt results, in part, from the frictional forces of the melt processing. The melt processing includes melt-blending of toner addenda into the bulk of the polymer.
The polymer may be made using a limited coalescence reaction such as the suspension polymerization procedure disclosed in U.S. Pat. No. 4,912,009 to Amering et al., which is incorporated in its entirety by reference herein.
Useful binder polymers include vinyl polymers, such as homopolymers and copolymers of styrene. Styrene polymers include those containing 40 to 100 percent by weight of styrene, or styrene homologs, and from 0 to 40 percent by weight of one or more lower alkyl acrylates or methacrylates. Other examples include fusible styrene-acrylic copolymers that are covalently lightly crosslinked with a divinyl compound such as divinylbenzene. Binders of this type are described, for example, in U.S. Reissue Pat. No. 31,072, which is incorporated in its entirety by reference wherein. Preferred binders comprise styrene and an alkyl acrylate and/or methacrylate and the styrene content of the binder is preferably at least about 60% by weight.
Copolymers rich in styrene such as styrene butylacrylate and styrene butadiene are also useful as binders as are blends of polymers. In such blends, the ratio of styrene butylacrylate to styrene butadiene can be 10:1 to 1:10. Ratios of 5:1 to 1:5 and 7:3 are particularly useful. Polymers of styrene butylacrylate and/or butylmethacrylate (30 to 80% styrene) and styrene butadiene (30 to 80% styrene) are also useful binders.
Styrene polymers include styrene, alpha-methylstyrene, para-chlorostyrene, and vinyl toluene; and alkyl acrylates or methylacrylates or monocarboxylic acids having a double bond selected from acrylic acid, methyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenylacrylate, methylacrylic acid, ethyl methacrylate, butyl methacrylate and octyl methacrylate and are also useful binders.
Also useful are condensation polymers such as polyesters and copolyesters of aromatic dicarboxylic acids with one or more aliphatic diols, such as polyesters of isophthalic or terephthalic acid with diols such as ethylene glycol, cyclohexane dimethanol, and bisphenols. Other useful resins include polyester resins, such as by the co-polycondensation polymerization of a carboxylic acid component comprising a carboxylic acid having two or more valencies, an acid anhydride thereof or a lower alkyl ester thereof (e.g., fumaric acid, maleic acid, maleic anhydride, phthalic acid, terephthalic acid, trimellitic acid, or pyromellitic acid), using as a diol component a bisphenol derivative or a substituted compound thereof. Specific examples are described in U.S. Pat. Nos. 5,120,631; 4,430,408; and 5,714,295, all incorporated herein by reference, and include propoxylated bisphenolxe2x80x94A fumarate, such as Finetone(copyright) 382 ES from Reichold Chemicals, formerly Atlac(copyright) 382 ES from ICI Americas Inc.
A useful binder can also be formed from a copolymer of a vinyl aromatic monomer; a second monomer selected from either conjugated diene monomers or acylate monomers such as alkyl acrylate and alkyl methacrylate.
An optional additive for the toner is a colorant. In some cases the magnetic component, if present, acts as a colorant negating the need for a separate colorant. Suitable dyes and pigments are disclosed, for example, in U.S. Reissue Pat. No. 31,072 and in U.S. Pat. Nos. 4,160,644; 4,416,965; 4,414,152; and 2,229,513, all incorporated in their entireties by reference herein. One particularly useful colorant for toners to be used in black and white electrostatographic copying machines and printers is carbon black. Colorants are generally employed in the range of from about 1 to about 30 weight percent on a total toner powder weight basis, and preferably in the range of about 2 to about 15 weight percent. The toner formulations can also contain one or more other additives of the type used in conventional toners, including magnetic pigments, colorants, leveling agents, surfactants, stabilizers, and the like.
The remaining components of toner particles as well as the hard magnetic carrier particles can be conventional ingredients. For instance, various resin materials can be optionally used as a coating on the hard magnetic carrier particles, such as fluorocarbon polymers like poly (tetrafluoro ethylene), poly(vinylidene fluoride) and poly(vinylidene fluoride-co-tetrafluoroethlyene). Examples of suitable resin materials for the carrier particles include, but are not limited to, silicone resin, fluoropolymers, polyacrylics, polymethacrylics, copolymers thereof, and mixtures thereof, other commercially available coated carriers, and the like.
When the toner formulation of the present invention is used in a single component toner system, the toner formulation has present charging particles as well, such as negatively charging particles. The amount of the charging particles for the single component optional system are conventional amounts. When a single component system is used, preferably the charging particles are at least one type of magnetic additive or material, such as soft iron oxide which is dispersed in the toner. Examples of useful charging particles include mixed oxides of iron, iron silicon alloys, iron aluminum, iron aluminum silicon, nickel iron molybdenum, chromium iron, iron nickel copper, iron cobalt, oxides of iron and magnetite. Other suitable magnetic materials that can be present in the toner include, but are not limited to, magnetic material containing acicular magnetites, cubical magnetites, and polyhedral magnetites. A useful soft iron oxide is TMB1120 from Magnox Inc.
The toner formulations of the present invention can also be used in magnetic image character recognition (MICR). In such an application, the amount of the magnetic material in the toner particles of the present invention can be any amount sufficient to preferably meet commercial needs, such as providing a sufficient signal strength for the toners developed as an image. Preferably, the amount of magnetic loading in the toner compositions is from about 40% to about 50% by weight of the toner particles, and more preferably from about 42% to about 45% by weight of the toner particles though other amounts can be used. The toner preferably comprises, based on the weight of the toner, from about 40 to about 60 wt % polymer; from about 30 to about 55 wt % magnetic additive or material; optionally from about 1 to about 5 wt % release agent; and the preferred concentrations of silicon dioxide described above, all based on the weight of the toner.
The present invention further relates to methods of forming images using the toners and developers of the present invention. Generally, the method includes forming an electrostatic latent image on a surface of an electrophotographic element and developing the image by contacting the latent image with the toner/developer of the present invention.
The present invention further relates to the use of the above-described development system in developing electrostatic images with the toner of the present invention. The method involves contacting an electrostatic image with the toner of the present invention. For example, the method involves developing an electrostatic image member bearing an electrostatic image pattern by moving the image member through a development zone and transporting developer through the development zone in developing relation with the charge pattern of the moving imaging member by rotating an alternating-pole magnetic core of a pre-selected magnetic field strength within an outer non-magnetic shell, which can be rotating or stationary, and controlling the directions and speeds of the core and optionally the shell rotations so that developer flows through the development zone in a direction co-current with the image member movement, wherein an electrographic two-component dry developer composition is preferably used. The dry developer composition contains charged toner particles and oppositely charged carrier particles. The carrier particles are preferably a hard magnetic material exhibiting a coercivity of at least about 300 gauss when magnetically saturated and also exhibit an induced magnetic moment of at least about 20 EMU/gm when in an externally applied field of 1,000 gauss. The carrier particles have a sufficient magnetic moment to prevent the carrier particle from transferring to the electrostatic image. The various methods described in U.S. Pat. Nos. 4,473,029 and 4,546,060 can be used in the present invention using the toner of the present invention in the manners described herein, and these patents are incorporated in their entirety by reference herein.
The electrostatic image so developed can be formed by a number of methods such as by imagewise photodecay of a photoreceptor or imagewise application of a charge pattern on the surface of a dielectric recording element. When photoreceptors are used, such as in high-speed electrophotographic copy devices, the use of half-tone screening to modify an electrostatic image is particularly desirable; the combination of screening with development in accordance with the method of the present invention producing high-quality images exhibiting high Dmax and excellent tonal range. Representative screening methods include those employing photoreceptors with integral half-tone screen, such as those described in U.S. Pat. No. 4,385,823, incorporated in its entirety by reference herein.
Developers in the development system of the present invention are preferably capable of delivering toner to a charged image at high rates and hence are particularly suited to high-volume electrophotographic printing applications and copying applications.
The present invention can be further clarified by the following examples, which are intended to be purely exemplary of the present invention.
The test apparatus for measuring rub-off from an image-bearing substrate having a first side and a second side with a toner image on the first side has a flat surface having a first and second end and adapted to support a first substrate with one of its ends extending beyond the first end of the flat surface (test sheet); a restrainer for preventing movement of the second substrate (receiver sheet) along the length of the flat surface; a pressure pad adapted to impose a selected pressure on the first substrate and the second substrate in a test area; a puller adapted to pull the first substrate a selected distance through the test area relative to the second substrate; a calibrated scanner; and, a computer program for converting the scanned results into a numerical test results. The test sheet is positioned with its first side against the receiver substrate. Any apparatus which is effective to move the image-bearing side of the test sheet an effective distance through a test area relative to the receiver sheet and in contact with the receiver sheet at a selected pressure is suitable.
The substrates tested are typically paper sheets. The test sheet is a paper sheet bearing on its first side a toner image. This sheet is positioned so that one of its ends extends beyond the first end of the flat surface for engagement and removal therefrom. The second sheet is then placed over the first sheet and fastened to restrain its movement relative to the flat surface. A pressure is then imposed on a test area typically near the first end of the flat surface. The first sheet is then pulled from the flat surface and the resulting toner rub-off in the test area is indicative of the rub-off from the test sheet.
Such an apparatus and test procedure are disclosed in U.S. patent application No. (unassigned), entitled xe2x80x9cRub-off Test Method and Apparatus,xe2x80x9d filed Mar. 13, 2001 by John R. Lawson, Gerard Darby II, and Joseph A. Basile, with Attorney Docket No. HEID-25,491, and this application is incorporated in its entirety by reference herein.
The test apparatus is designed to move the test sheet through a test area subject to a test pressure for a selected distance relative to the receiver sheet to determine the rub-off tendencies of the test sheet. It will be understood that the apparatus could operate with the test sheet above the receiver sheet so long as the test sheet is moved relative to the receiver sheet.
The measurement of rub-off is accomplished in two steps. The first step is to abrade the test sheet images on a suitable apparatus. The second step is to take the results of the abrasion test and analyze the results to obtain a quantitative measure of the rub-off characteristics of the test sheet.
The first step of generating the test sheets is accomplished by producing the test sheets on the system to be evaluated. The test prints for rub-off are desirably made up with text printed over the entire imaging area of an 8.5xc3x9711 inches sheet. A representative test sheet (target) is prepared. Desirably, the text is written on the test sheet at a suitable angle (i.e., seven degrees) relative to the horizontal. This is to eliminate streaks in the final image where breaks between words exist. In typical use, this target is rendered as a postscript file and sent to the printer. The printer then uses this input file to generate test sheets for evaluation under specific test conditions. Typically a standard paper, such as Hammermill Bond, is used for test-to-test consistency.
Once the test sheets have been made on the printer under study, the evaluation samples are made. These are generated by rubbing the test sheets (Hammermill Bond or any other standard paper) against the receiver sheets in a controlled manner. This control is obtained through the use of the apparatus described above
To use the apparatus, the following steps are followed:
1. The test sheet is placed on the flat surface, face up. The sheet is aligned to a registration mark so that the leading edge of the test sheet protrudes beyond the first end of the flat surface.
2. The receiver sheet (second sheet) is placed on the test sheet. The receiver sheet is aligned with the first end of the flat surface. The other end of the receiver sheet is clamped in place.
3. A known weight is then placed in a holder and rests on the paper stack. The weight provides a known pressure on the stack in a test area. In these experiments, 3PSI was used.
4. The flat surface is then moved laterally until the leading edge of the test sheet engages a roller nip. The rollers turn and xe2x80x9cgrabxe2x80x9d the test sheet and pull it out from under the receiver sheet at 21 inches per second. The relative motion between the test sheet and the receiver sheet causes the toner from the test print to be abraded by the receiver sheet in the test area. This results in a xe2x80x9ctoner smearxe2x80x9d image on the receiver sheet. The level of xe2x80x9csmearingxe2x80x9d in the test area has been shown to correlate with the subjective measure of rub-off.
5. Steps 1 to 4 are repeated six times. The replicates may be handled in one of two ways. In the first method all six replicates are done with a selected pressure from about 0.5 to about 5 pounds per square inch (PSI). In the second method, two samples are made at each of three pressures, such as 1, 2, and 3 PSI. The differences in the analysis of the two methods are given in the next section.
To analyze the test sheets, the following procedure is followed:
1. Each test area is scanned on a calibrated scanner. The scanner is calibrated as follows:
a) a step tablet of known density is scanned using the same scan conditions as used when the print is scanned;
b) the contrast and zero point of the scanner are adjusted so that the digital values for the step tablets are at a predetermined value, within limits; and,
c) the values of the step tablet are periodically checked when doing many scans (e.g., once an hour).
2. With the calibrated scanner, the six images from each test area are scanned. The scan options are selected to give the six scanned test areas sequential names. The scans are 230xc3x97230 pixels at 600 dots per inch in grayscale mode. The scanned test area is stored on the file server.
3. The data in the scanned files represent the luminance of the pixels in the scanned area. 0=black and 255=white. For each test area, the standard deviation of the luminance values is calculated. Standard deviation has been shown to provide a measure with a good signal-to-noise ratio that correlates with subjective evaluations of rub-off.
4. If all six test areas were made using the same weight, the standard deviation values for luminance are averaged and the average value is reported as the rub-off for the sample under test.
5. If the six test areas are made using three weights, the six standard deviation values are regressed against the pressures at which they were tested. A least squares regression curve, preferably a second order linear regression, is fit through this data and the estimated values for rub-off at predetermined pressures are calculated. These rub-off values as a function of pressure are the results reported for the test.
6. Confidence limits on the reported values are calculated for both data analysis methods and are typically +/xe2x88x9210% of the rub-off value.
A wide variety of apparatus can be used to maintain a pressure pad bearing a weight to produce the desired pressure in the test area in position. Basically, the pressure pad must be maintained in position so that it can exert the desired pressure on the top of the second sheet while being retained in position relative to the flat surface when either of the sheets is moved. This is can be accomplished by a variety of mechanical configurations. Such variations are obvious to those skilled in the art.